The atomic force microscope (AFM) continues to fi nd increasing applications in nanoscale imaging, [ 1 ] metrology, [ 2 ] devices, [ 3 ] and manufacturing. [ 4 ] In these applications, tip size and shape critically affect the accuracy, resolution, and reliability of measurements and processes. [ 5 ] However, during tip-sample contact the tip can wear and break, undermining the utility of the instrument. [ 6 ] Thus, the development of wear-resistant probes, protocols for their testing and a fundamental understanding of their wear process is of vital importance. Although wear-resistant probes continue to advance, [ 7 , 8 ] tribological test methods and the collection of data for theoretical models still require measurements ex situ to the scanning process. Scanning electron microscopy (SEM) [8][9][10][11][12] and blind reconstruction while scanning on highaspect-ratio reference samples [ 8 , 12 , 13 ] have both been used to correlate scanning history with tip geometry changes. Such ex-situ approaches are slow and can cause additional wear, fracture, and contamination. More recently, periodic forcedisplacement adhesion measurements have provided a less disruptive means of monitoring changes in contact area after scanning a fi nite distance. [ 8 , 11 , 12 ] Still, adhesion measurements require interruption of the scan, and quantitative determination of a contact radius can be strongly affected by geometry, contamination, and environmental conditions. Here, we demonstrate how contact resonance force microscopy (CR-FM) methods enable quantitative in-situ evaluation of tip wear by measurement of instantaneous changes in contact radius while scanning Si cantilevers on a Si substrate. It is found that CR-FM measurements do not adversely affect the wear process, and the results compare favorably with ex-situ techniques. Overall, CR-FM is shown to be an effective tool for detecting subnanometer changes in the contact radius while also revealing novel information about tip symmetry and wear rate.
Contact resonance force microscopy experiments and analysis are well described in the literature. [ 14 ] Briefl y, an AFM tip is brought into contact with a sample, and the sample or cantilever is vibrated out of plane over a frequency range that excites a fl exural resonance. Due to tip-sample interactions, the contact resonance of a given eigenmode